Title: L' Nogach, IHEP, Protvino for the STAR Collaboration
1Measurements of transverse spin effects in the
forward region with STAR detector
L. Nogach, IHEP, Protvinofor the STAR
Collaboration
- Outline
- Motivation
- STAR detector
- Inclusive p0 measurements
- More possibilities with the FMS
- 2009 run at vs500 GeV
- Summary and outlook
2Motivation
- Contrary to simple pQCD predictions, first
measurements of ? production in p?p collisions
found large transverse single spin asymmetries.
Similar large effects have recently been found in
ee- and semi-inclusive deep-inelastic
scattering. - Significant developments in theory in the past
few years suggest common origins for these
effects, but large transverse spin asymmetries in
p?p ? ? X production are not yet fully
understood. - More fundamental question what is the underlying
dynamics in kinematics where transverse spin
effects are observed?
3Polarized pp collisions at RHIC
RHIC pC Polarimeters
Absolute Polarimeter (H? jet)
Siberian Snakes
Siberian Snakes
PHENIX
STAR
Spin Rotators (longitudinal polarization)
Spin Rotators
Pol. H- Source
LINAC
BOOSTER
Helical Partial Siberian Snake
AGS
200 MeV Polarimeter
AGS pC Polarimeter
FOMP2L
Strong AGS Snake
4STAR detector layout
FMS
TPC
FPD
BBC
BEMC
- FPD
- Modular detector
- Small xF-pT range compared to FMS
- FMS
- 20x acceptance of previous forward detectors at
STAR - Full azimuthal coverage for 2.5 lt ? lt 4.0
- Array of 1264 Pb-glass cells
FPD
FMS
5Published measurements with the FPD (2002?2006)
PRL 92, 171801 (2004)
- PRL 101, 222001 (2008)
- AN at positive xF grows with increasing xF
- xF dependence matches theoretical model
expectations qualitatively - pT dependence at fixed xF is not consistent with
1/pT expectation of theoretical model calculations
vs200 GeV, lt?gt 3.8
U.DAlesio, F.Murgia Phys. Rev. D 70, 074009
(2004) arXiv0712.4240 C.Kouvaris, J.Qiu,
W.Vogelsang, F.Yuan, Phys. Rev. D 74, 114013
(2006)
Asymmetry revealed at lower energies persists at
vs200 GeV
6Inclusive h AN measurements (2006)
Asymmetry for h-mesons is larger than for p0
(similar to the E704 measurements) (AN)h
0.361 0.064 for 0.55ltxFlt0.75
arXiv0905.2840
7Inclusive p0 AN with the FMS (2008)
arXiv0901.2828
Octant subdivision of FMS for inclusive p0 spin
sorting
- Azimuthal dependence appears
- to be as expected
- AN is comparable to prior measurements with the
FPD
8AN(pT) at xFgt0.4 with the East FPD/FMS (2008)
positive xF
- AN for negative xF consistent with zero
- Indication of AN for positive xF persists
- up to pT 5 GeV/c
- Needs more transverse spin running
arXiv0901.2763 A.Ogawa _at_CIPANP09
9High xF vector mesons
3 photon events to look for w?p0g (BR 8.9)
- pT(triplet)gt2.5 GeV/c
- E(triplet)gt30 GeV
- pT(photon cluster)gt1.5 GeV/c
- pT(p0)gt1 GeV/c
Background only MC Run8 FMS data Fit is Gaussian
p3
µ0.7840.008 GeV s0.0870.009
GeV Scale1339135 Events
Significant (10s) w?p0g signal seen in the data
gt
possibility to measure spin-1 meson AN
arXiv0906.2332
10First look at jet-like events in the FMS
Event selection
Jet-shape distribution of energy within
jet-like objects in the FMS as a function of
distance from the jet axis
- gt15 cells with energy gt 0.4GeV in the event
- (no single pions in the event)
- cone radius 0.5 (?-f space)
- Jet-like pT gt 1 GeV/c, xF gt 0.2
- 2 perimeter fiducial volume cut (small/large
cells)
- Jet shape in data matches simulation well
- Reconstructed mass does not match
- as well
- High-tower trigger used in Run 8 biases
- jets
arXiv0901.2828
11Forward p0-p0 azimuthal correlations
- Possible back-to-back di-jet/di-hadron Sivers
measurement - Possible near-side hadron correlation for
Collins fragmentation function/ - Interference fragmentation function
Transversity - Low-x / gluon saturation study accessing
lowest xBjgluon
12Benchmark for Drell-Yan
First look at J/? ? e e- in the FMS
Reconstructed 2-cluster invariant mass
- Fit with Gaussian Offset
- Gaussian fit parameters
- µ 3.080 0.020 GeV/c2
- s 0.082 0.026 GeV/c2
- ?2/d.o.f. 20.83/26
- Significance from the fit
- 4.5 s
-
High-xF, high mass dilepton pairs are difficult
to reconstruct
- Cuts applied
- E_pair gt 60.0 GeV
- z?? lt 0.7
- Isolation radius 0.4 ?-f
- pT_cluster gt 1.0 GeV/c
First high-xF J/? measurement at vs gt 62 GeV
arXiv0907.4396
132009 first physics run at vs500 GeV
Luminosity 56x56 bunches with 1.8x1011
intensity 10 pb-1 integrated Polarization 30
(longitudinal)
Forward Pion Detector module schematic
Shower Maximum Detector
lead converter
7x7 matrix of lead glass cells
preshower (7 Pb-glass cells)
fit by Gaussiangamma function
- First look at po event reconstruction in the FPD
- using matrixpreshower (no SMD data)
- 20 GeV lt Etotal lt 80 GeV
- fixed vertex (z0), no minbias condition
- Ng2
142009 data at vs500 GeV
FPD measures energy up to 200 GeV -gt SMD
information is required to reconstruct pions
above 60 GeV
Example of 2-photon event when two clusters
significantly overlap in the matrix, but are
clearly separated in the SMD
15Forward Hadron Calorimeter
- Proposed to be staged at a minimal Dz from the
FMS symmetrically of DX magnet - two matrices of 9x12 Pb-scintillator detectors
- real jets physics with
- FMSFHC
- polarization transfer coefficients
- through measurement of
- ? polarization
- in the ??n?0 channel
first simulations of L?np0
16Summary
- Large transverse single spin asymmetries at large
h are observed up to vs200 GeV. - High precision inclusive p0 AN measurements with
the FPD allow for a quantitative comparison with
theoretical models. - FMS allows to look at heavier mesons, jet-like
events and particle correlations. - Essential to go beyond inclusive production to
disentangle dynamical origins.
17Outlook
- Measurements of AN in inclusive p0 production at
vs500 GeV - (Run 11?).
- Extend forward calorimetry to have hadronic
capability (FHC) to measure full jets and to
study inclusive L production. - Extend measurements of transverse single spin
asymmetries from hadron production to prompt
photon and jets. - Develop RHIC experiment for a future measurement
of transverse single spin asymmetries for
Drell-Yan production of dilepton pairs.
18Backups
19Single Spin Asymmetry
- Definition
- ds?(?) differential cross section of p0
when incoming proton has spin up(down)
- Two methods of measurements
- Single arm calorimeter
-
- R relative luminosity (by BBC)
- Pbeam beam polarization
- Two arm (left-right) calorimeter
- No relative luminosity needed
positive AN more p0 going left to polarized beam
20Possible mechanisms
- Sivers effect Phys. Rev. D 41, 83 (1990) 43,
261 (1991) - Flavor dependent correlation between the
proton spin (Sp), proton momentum (Pp) and
transverse momentum (kT) of the unpolarized
partons inside. The unpolarized parton
distribution function fq(x,kT) is modified to - Collins effect Nucl. Phys. B396, 161 (1993)
- Correlation between the quark spin (sq),
quark momentum (pq) and transverse momentum (kT)
of the pion. The fragmentation function of
transversely polarized quark q takes the form
According to the latest theoretical developments,
both mechanisms contribute to ?? AN
21Separating Sivers and Collins effects
Collins mechanism asymmetry in the forward jet
fragmentation
Sivers mechanism asymmetry in the forward jet or
g production
SP
SP
kT,q
p
p
p
p
Sq
kT,p
Sensitive to proton spin parton transverse
motion correlations
Sensitive to transversity
To discriminate between the two effects we need
to go beyond p0 detection to direct photons or
jet-like events
22p0 inclusive cross section
Cross-section is consistent with NLO pQCD
calculations
PRL 97, 152302 (2006) nucl-ex/0602011
23Forward p0 midrapidity azimuthal correlations
- pQCD inspired GSV cuts (Guzey, Strikman and
Vogelsang, hep-ph/0407201) - ?TPC lt 0.9 2.8 lt ?FMS lt 3.8
- 2.5(2.0)GeV lt pTFMS
- 1.5(1.0)GeV lt pTTPC lt pTFMS
- z??FMSlt 0.7, 0.07 lt M?? lt 0.30 GeV
- only leading particle considered,
- corrected for pile-up
- as proposed in hep-ex/0502040
- Possible back-to-back di-jet/
- di-hadron Sivers measurements
- Low-x / gluon saturation study
- Step-stone towards transverse
- spin forward photon-jet
arXiv0907.3473
24Future runs at RHIC